Electrostatic method and apparatus using non-conductive discharge means



Nov. 3, 1959 s. R. PRANCE 2,911,323

ELECTROSTATIC METHOD AND APPARATUS USING NON-CONDUCTIVE DISCHARGE MEANS 1 Filed Nov. 29, 1955 Pam 5e 60/2 4 Y 76 Pom 5e 50pm Y IN V EN TOR.

United States Patent O ELECTROSTATIC METHOD AND APPARATUS 55158 NON CONDUCTIVE DISCHARGE Stanley R. Prance, D ayton, Ohio, assignor to General Motors Corporation, Detroit, Mich., a corporation of Delaware I Application November 29, 1955, Serial No. 549,719

' Claims. (Cl. 117-93) This invention relates to an electrostatic coating method and apparatus and is particularly concerned with electrostatic painting or covering operations wherein the paint is applied at substantially atmospheric pressure to the spray head.

It is an object of the invention to provide an apparatus and-method for the electrostatic deposition of paint wherein the paint is fed, by gravity,-to a stationary nonmetallic discharge electrode member, or spray head, and is atomized from said member onto the part to be painted. I

A still further object of the invention is to provide electrostatic painting equipment which does not involve moving parts and wherein the painting pattern may be controlled by controlling the shape and disposition of the spray head.

A still further object of the invention is to provide an apparatus and method for painting objects electrostatically wherein paint is supplied to a spray head at substantially atmospheric pressure and is distributed in an atomized mist therefrom, wherein the directivity of the atomized paint may be readily controlled by the shape and/or disposition of the spray head. i

In carrying out the above object it is a still further object of the invention to feed the paint to the surface of the spray head at a point intermediate the power source and the atomizing edge of the spray head.

Further objects and advantages of the Present inven' minal thereof. The other terminal of the power supply tion will be apparent from the following description, reference being had to the accompanying drawings, wherein preferred forms of the invention are clearly shown.

In the drawings:

Figure l is a diagrammatic view of a spray head used to paint a steering wheel which is shown for illustrative purposes on a conveyer;

Figure 2 is a fragmentary sectional view taken on line 22 of Figure l; and

Figure 3 is a plan view of the spray head with the paint supply means andthe power supply means removed.

Electrostatic paint spraying operations are well known in the art. These operations generally comprise atomizing directly from air pressurized spray guns which direct paint to the objects to be painted wherein the objects to be painted are maintained within an electrostatic field whereby the paint is directed to the object more uniformly and economically than is possible with conventional paint spraying methods.

This type of spray painting equipment and method is described and shown in Ransburg et al. Patent 2,247,963.

Several new developments in electrostatic paint spraying have been made during the past few years and one of these is disclosed and claimed in copending application S.N. 527,723 assigned to the assignee of the present invention. In this'instance a stationary relatively wide spray head is utilized which comprises a fiat metal plate onto which paint is supplied at substantially atmospheric pressure. The plate forms one side of the electrostatic "ice circuit while the work to be painted forms the other side thereof. As the paint passes down the spray head or plate by gravity it is charged so that upon leaving the plate it is atomized into a mist which is attracted to the surface of the work. The present invention is directed to an improvement in the method and apparatus for spray painting articles electrostatically as is disclosed in said copending application. The improved apparatus is showndiagrammatically in Figure 1 wherein the spray head or electrode, as it may be termed, is designated at 20. This discharge electrode is a substantially flat plate-like member and is formed from nonmetallic material such as Bakelite, Formica, polystyrene, hard rubber, etc. The plate is of a generally triangular shape as shown in Figure 3 and is relatively thin, preferably in the order of to A of an inch. The front edge or'discharge edge of the plate is relatively narrow and may be rounded as at 22 forming a relatively narrow section at the discharge end of the plate 20. The sides of the plate diverge from this rounded front edge at an acute angle and preferably at an angle'of between 12 and 30. The discharge end of the plate 22 should be relatively narrow and the transverse dimension taken normal to the main axis of the plate at this point should be no more than 1 /2 to 2 inches for most applications of the device and no less than of an inch although it is apparent that these dimensions may vary in accordance with the shape and size of the part to be sprayed. When spraying a steering wheel, for example, the dimensions set forth are entirely adequate. The width of the discharge end of the plate 20 and the shape, radius or rounded contour thereof may be varied to control to some extent the shape and size of the paint spray pattern. Suchcontrol is best arrived at by experimentation for the specific part to be sprayed.

The plate 20 is mounted adjacent the rear portion thereof on a threaded stud 24 which is secured to the plate 20 by means of nuts 26 and 28. The stud 24 is hingedly connected at 30 to a standard 32, all of said connections being metallic. The standard 32 connects to a power supply 34 and preferably to the negative teris grounded. Intermediate the stud 24 and the discharge end 22 of the plate is disposed a paint supply tube 36. This tube is centrally located of the plate axially thereof and may be metallic or nonmetallic, preferably the latter,

with its orifice facing the discharge end of the plate. To the tube 36 is connected a rubber hose or other suitable conduit 38 that attaches to a paint supply 40. The paint spray 40 may be maintained above the plate 20 so that the paint flows by gravity alone or a pressurized tank may be used wherein air is supplied to the tank 40 through a conduit 42 in sufficient quantities to force the paint from the tank 40 and through the discharge tube 36 onto the surface of the plate 20 at substantially atmospheric pressure. No attempt should be made to cause the paint to flow under pressures substantially above atmospheric pressure from the paint supply tube 36. The object to be painted, in this instance a wheel 50, is suspended on a hanger 52 that is carried by a conveying means 54 along the track 56. The track 56, the conveying means 54 and the hanger 52 are all metal as is the spider which is normally present within the wheel 50. In this instance the steering wheel 50 has a rubberlike covering over the metallic reinforcing spider and it is this covering which is to accept the paint during the painting process. The conveying means 54 is moved along the track 56 by a chain 58 that is attached to a motivating source, not'shown. The means 54 may include an arm 60 thereon which actuates on and off switches 62 and 64 mounted on the track 56. In this manner it is possible to connect the switches in circuit at the power supply 34 so that the power supply is turned on and off as the parts to be spray painted have moved through the field of spray. This electrical control prevents loss of paint although the on and ofi feature forms no part of this invention.

It will be noted that the spray heador plate 24) is preferably positioned at an angle to the horizontal. This angle should be between and 30 from the horizontal to obtain best results. When the angle exceeds 30 the paint flowing from tube 36 moves too rapidly down the plate whereas when the angle is less than 5 the flow is too slow to provide satisfactory operation. A preferred angle is in the order of to from the horizontal. This angle obviously controls to some extent the quantity of paint sprayed since it controls the rate of gravity fiow of the paint on the plate. In this connection the inside diameter of the paint supply tube may also be varied to control the quantity of paint delivered to the discharge edge of the plate 20. It is desirable to maintain the paint whichflows down the plate 20 in a relatively thin film thereon which is determined as is well known in the art by the quantity of solvent or thinner used in the paint mixture. Paints having viscosities of from 13 to 21 seconds 4 Ford 01p 83 F.) give excellent results with the discharge plate disclosed.

The basic difference between this invention and the invention disclosed in said copending application S.N. 527,723 resides in the paint supply plate 20 which in this instance is nonmetallic together with the specific disposition of the paint supply tube 36 with respect to the edge portion of the plate and the power supply means. I have found that the tube 36 should be substantially equidistant between two points for optimum results and in all cases should be between the spray edge and the power supply. It will be noted in said copending application that the powcrsupply to the plate is attached intermediate the spray edge thereof and the point at which the paint supply tube first contacts the plate. The present disposition of parts tends to yield more uniform spray results although the device of the copending application will function satisfactorily. Furthermore, it appears that the nonmetallic spray plate gives more uniform and desirable results than does a metal plate. The explanation for this improvement with respect to the several parts as described herein is not entirely understood but actual tests have proved that the nonmetallic plate in substantially fiat form maintained at an angle as noted and having a relatively narrow discharge edge is more easy to control, dispenses paint in a more uniform spray and in general yields improved results over any of the electrostatic spray heads heretofore noted.

While the forms of embodiment of the invention as herein disclosed constitute preferred forms, it is to be understood that other forms might be adopted, as may come within the scope of the claims which follow.

What is claimed is as follows:

1. In a method for electrostatically painting the surface of an article with a paint, the steps comprising; providing a relatively flat, elongated plate of electrically nonconductive material, spacing said plate remote from p the article to be painted, positioning said plate at a downward angle of not more than from the horizontal, impressing a high voltage DC. current across said plate and said article, and then flowing paint down the inclined plate from a stationary 'point substantially equidistant from the ends thereof by gravity alone, whereby the paint is atomized from the lower edge of the plate solely by the electrostatic fieldforrned between the plate and the article to be painted.

2. In a method for electrostatically painting the surface of an article with paint, comprising the steps of,

providing a relatively flat, elongated plate of electrically nonconductive material, positioning said plate at a downward angle of not more than 30 from the horizontal, spacing the article to be painted from the lower edge of said plate, attaching one terminal of a high voltage D.C. source adjacent the high end of said inclined plate, attaching the other terminal of said high voltage D.C. source to the article to be painted, supplying flowable paint to the plate at a stationary point intermediate the upper and lower edges thereof for causing the paint to flow down the plate by gravity alone, whereby said paint is atomized as it leaves the lower edge of the plate the solely to the electrostatic field between the plate and the article for painting the article.

3. An electrostatic spray head for use in electrostatic spray painting operations comprising an electrically nonconductive flat plate of generally truncated triangular shape wherein the base of the triangle is relatively wider than the truncated end thereof, supporting means attached to the plate adjacent the base portion thereof for supporting the plate in an angled position with respect to the horizontal plane wherein the truncated end of the plate is lower than the supported end and wherein the angle of the plate from the horizontal does not exceed 30", a stationary paint supply means contacting the plate initially adjacent the central portion thereof and having an outlet adjacent the central portion of the plate and capable of supplying flowable paint onto the inclined surface of the plate, and an electrical connection to the plate adjacent .the base portion thereof adapted to be connected to a high voltage D.C. source.

4. A spray head for an electrostatic spray painting apparatus comprising in combination; a nonmetallic and electrically nonconductive plate having a generally triangular shape wherein the angle between the sides thereof is between 12 and 30, a support for the plate adapted to be attached adjacent the base of the generally triangular plate and capable of supporting the plate at a downward angle from the horizontal between 10 and 30, stationary paint supply means initially contacting the plate ,at a point intermediate the point of support and the truncated edge thereof and including an outlet for paint whereby paint supplied to the plate will flow downwardly of the plate and over the lower edge thereof and an electrical connection on the plate adapted to be connected to a power source.

5. A spray head for electrostatic paint spray equipment comprising a relatively flat, generally truncated, triangular-shaped, electrically nonconductive plate ,having a base end and an opposed discharge end of less dimension than the base end, a support for said plate attached to the plate adjacent the base and disposed so that the end of the plate opposite to the base end is depressed from the horizontal and at an angle not exceeding 30, paint supply means having a stationary outlet on the upper surface of the plate intermediate the two ends thereof, and an electrical connection, for connecting the plate to a ,power source, disposed adjacent the base end of the plate and between the base end of the plate and the paint ,supply means.

References Cited in the file of this patent UNITED STATES PATENTS 1,846,845 Clark Feb. 23, .1932 1,968,279 Bracewel-l July 31, 1934 2,048,651 Norton July 21, 1936 2,680,079 Huebner June 1, 1954 2,684,656 Ransburg July 27, 1954 2,781,279 Miller Feb. 12, 1957 2,785,088 Ransburg Mar. 12,1957

FOREIGN PATENTS 710,852 Great Britain June .23, 19.54 

1. IN A METHOD FOR ELECTROSTATICALLY PAINTING THE SURFACE OF AN ARTICLE WITH A PAINT, THE STEPS COMPRISING; PROVIDING A RELATIVELY FLAT, ELONGATED PLATE OF ELECTRICALLY NONCONDUCTIVE MATERIAL, SPACING SAID PLATE REMOTE FROM THE ARTICLE TO BE PAINTED, POSITIONING SAID PLATE AT A DOWNWARD ANGLE OF NOT MORE THAN 30* FROM THE HORIZONTAL, IMPORESSING A HIGH VOLTAGE D.C. CURRENT ACROSS SAID PLATE AND SAID ARTICLE, AND THEN FLOWING PAINT DOWN THE INCLINED PLATE FROM A STATIONARY POINT SUBSTANTIALLY EQUIDISTANT FROM THE ENDS THEREOF BY GRAVITY ALONE, WHEREBY THE PAINT IS ATOMIZED FROM THE LOWER EDGE OF THE PLATE SOLELY BY THE ELECTROSTATIC FIELD FORMED BETWEEN THE PLATE AND THE ARTICLE TO BE PAINTED 